IEEE 802.15.4 LR-WPAN Device Architecture
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The device comprises a physical layer (PHY), which contains the RF transceiver along with its low-level control mechanism. A MAC sublayer provides access to the physical channel for all types of transfer. The upper layers consist of a network layer, which provides network configuration, manipulation, and message routing.The application layer provides the intended function of a device. An IEEE 802.2 logical link control (LLC) can access the MAC through the service specific convergence sublayer (SSCS).

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1. Physical Layer

  • The PHY provides two services: the PHY data service and PHY management service interfacing to the physical layer management entity (PLME).
  • The PHY data service enables the transmission and reception of PHY protocol data units (PPDUs) across the physical radio channel.
  • The PHY management services are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium.
  • There are three frequency bands: 2.4 GHz global, 915 MHz America and 868 MHz Europe. There is a single channel between 868 and 868.6 MHz, 10 channels between 902 and 928 MHz, and 16 channels between 2.4 and 2.4835 GHz.
  • The data rate is 250 kbps at 2.4 GHz, 40 kbps at 915 MHz, and 20 kbps at 868 MHz Receiver sensitivities are -85 dBm for 2.4 GHz and -92 dBm for 868/915 MHz.The range of 10-20 meters is covered.
  • To maintain a common simple interface with MAC, both PHY data service and management service share a single packet structure.
  • A typical packet size for home applications is of the order of 30-60 bytes. More demanding applications such as interactive games require larger size packets. (127 bytes maximum)
  • The maximum packet durations are 4.25 ms for the 2.4 GHz band, 26.6 ms for the 915 MHz band, and 53.2 ms for the 868 MHz band.
  • The standard provides two options based on the frequency band. Both are based on direct sequence spread spectrum (DSSS).
  • The 868/915 MHz PHY uses a simple DSSS approach in which each transmitted bit is represented by a 15-chip maximum length sequence with BPSK modulation.
  • The 2.4 GHz PHY uses a 16-ary quasi-orthogonal modulation technique based on DSSS methods with OQPSK modulation.

2. Data link Layer

  • The data link layer is divided into two sublayers, the MAC and LLC sublayers.
  • The logical link control is standardized in IEEE 802.2 and is common among all IEEE 802 standards.
  • The MAC provides services to an IEEE 802.2 type logical link control through the service-specific convergence sublayer (SCCS), or a proprietary LLC can access the MAC services directly without going through the SCCS.
  • The SCCS ensures compatibility between different LLC sublayers and allows the MAC to be accessed through a single set of access points.
  • The features of the MAC are association and disassociation, acknowledged frame delivery, channel access mechanism, frame validation, guaranteed time slot management, and beacon management.
  • The MAC provides two services to higher layers that can be accessed through two service access points (SAPs). The MAC data service is accessed through the MAC common part sublayer (MCPS-SAP), and the MAC management services are accessed through the MAC layer management entity (MLME-SAP). These two services provide an interface between the SCCS or another LLC and the physical layer.
  • An important function of the MAC is to confirm successful reception of a received frame. Successful reception and validation of a data or MAC command frame is confirmed with an acknowledgment.
  • Three levels of security are provided: no security of any type; access control lists (non-cryptographic security); and symmetric key security, using AES-128.

3. Network Layer

  • The network layer of Zigbee (IEEE 802.15.4) is responsible for topology construction and maintenance as well as naming and binding services, which include the tasks of addressing, routing, and security.
  • IEEE 802.15.4 supports multiple network topologies, including star, peer-to-peer, and cluster tree. The topology is an application design choice.
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